This invention relates generally to control valves and, more specifically, to control valves for the precise metering of gaseous media between two substantially parallel surfaces in such a way that the volumetric flow rate is linearly proportional to the pressure differential across the metering surfaces. This flow condition is typically referred to as laminar flow.
The laminar flow volumetric flow rate/pressure differential relationship is described by the Hagen-Poiseuille Law as follows: ##EQU1## where Q=volumetric flow rate
K=constant PA0 h=distance between the plates, PA0 .mu.=viscosity, and PA0 .DELTA.P=pressure differential across the metering surfaces. PA0 Re=Reynolds Number PA0 U=average velocity PA0 .delta.=gap between the parallel surfaces PA0 .rho.=fluid density, and PA0 .mu.=viscosity
This flow description is valid for relatively narrow flow passages and/or very small velocities. A measure of how applicable the Hagen-Poiseuille Law is to a given flow pattern is the Reynolds Number EQU Re=U.rho..delta./.mu.
where
The critical Reynolds Number defining the transition between laminar and turbulent flow is approximately 2000. Therefore, the Hagen-Poiseuille Law is applicable for flow patterns having Reynolds Numbers of less than 2000. A typical Reynolds Number for a flow pattern of the type in which the present invention may be applied is approximately 5. This is clearly in the laminar flow region described by the Hagen-Poiseuille Law.
The prior art shows a number of devices for restricting the flow of liquid or gaseous media through a very narrow opening to produce a laminar flow pattern. U.S. Pat. No. 3,144,879 to Hans D. Baumann shows a device which uses two parallel surfaces separated by a distance which may be adjusted manually or as a function of the thermal expansion of a selected material. In the Baumann device, fluid enters through an input port into an annular channel which is sealed by two concentrically positioned O-rings. The O-rings are compressed between the upper surface of the main housing of the device and an adjustable plate which parallels this surface. An outlet port is provided just inside the outer O-ring seal to provide the fluid with a means of escape. The relatively narrow opening formed between the upper surface of the main housing and the adjustable plate in the area between the O-ring seals is effective to produce a laminar flow pattern through the device. The narrow gap may be adjusted by means of an adjusting screw which is threaded through the adjustable plate. A hand wheel is provided for turning the screw. Adjustment may also be attained by employing the force created by the difference in thermal expansion between two dissimilar metal parts.
U.S. Pat. Nos. 4,320,778; 4,322,055; and 4,278,234, also to Baumann, show variations of the device described in U.S. Pat. No. 3,144,879. The '234 patent shows a laminar flow restriction device in which the controlling gap between the metering surfaces is adjusted by a hydraulic amplifier interspaced between conventional linear motion-type pneumatic or hydraulic actuators. The '778 patent is a continuation-in-part of the '234 patent in which the laminar flow restriction device is adapted for use as an automatically operated small flow control valve. The '055 patent is also a continuation-in-part of the '234 patent, and describes additional embodiments of automatically operated small flow control valves which utilize the basic principles discussed in the preceding patents.
Other patents which show precisely regulating metering or control valves include U.S. Pat. Nos. 1,964,300; 1,983,213; 4,283,041; 4,552,336; 4,666,126; and 4,741,510. Also of interest to the particular embodiments illustrated and discussed below are U.S. Pat. Nos. 4,463,332 and 4,767,097 for an electrically operated proportional solenoid which are assigned to the assignee of the present invention.
An object of the present invention is to provide a laminar flow control valve of the type in which flow is throttled between two parallel surfaces, and in which the parallel surfaces are provided with a "self-aligning" feature.
Another object of the present invention is to provide a laminar flow control valve which may be easily and accurately controlled, calibrated and adjusted.
Yet another object of the present invention is to provide a laminar flow control valve which may be controlled, calibrated and adjusted by varying an electrical current, or by mechanical means without disassembly or direct access to the internal components of the valve.
Still yet another object of the present invention is to provide a laminar flow control valve having decreased sensitivity to line pressure variations.
These and other objects of the invention are attained in a control valve for precisely metering gaseous media which comprises a valve housing having an inlet and an outlet, an orifice plate, a poppet, an electrically-powered operating mechanism for positioning the poppet relative to the orifice plate, and a mechanism for aligning (or realigning) the poppet and the orifice plate when the valve is closed. The gaseous media is throttled between two parallel surfaces which define a portion of a flow path through the valve such that the volumetric flow rate is linearly proportional to the pressure differential across the parallel surfaces. The orifice plate is sealingly disposed in the flow path between the inlet and the outlet. The plate has a centrally located orifice and a first surface extending outwardly along the plate from the orifice. The poppet is disposed adjacent the orifice plate, and has a surface which extends adjacent and substantially parallel to the surface of the orifice plate. The operating mechanism positions the poppet relative to the orifice plate to open, close and meter the flow of the gaseous media through the valve by controlling the width of the gap between the two parallel surfaces. The valve is constructed so as to allow the surfaces to "self-align" each time the valve is closed so as to maintain the surfaces in a substantially parallel relation.
In the specific embodiment described in detail, the orifice plate is fixedly mounted in the flow path, and the poppet is attached to the operating mechanism such that the metering surface of the poppet is free to move in more than one plane relative to the metering surface of the orifice plate. In the preferred embodiment, the orifice plate comprises a substantially flat, disc-like plate sealingly mounted to the valve housing along its periphery. An orifice is formed in the approximate center of the plate. In this embodiment, the poppet has a substantially flat, disc-like portion disposed adjacent the orifice plate, and is attached to a movable stem extending from the operating mechanism. The poppet is attached to a spherical portion of the stem and is free to swivel relative to the stem and orifice plate. The swivel arrangement includes a spring disposed between the poppet and the spherical portion of the stem which tends to maintain the metering surface of the poppet in parallel relation to the metering surface of the orifice plate when the poppet is moved from a closed position to an open or metering position.
The preferred operating mechanism comprises an electrically powered proportional solenoid. The use of an electrically powered operator to precisely control the positioning of the poppet relative to the orifice plate offers advantages in terms of control and calibration of the valve.
The valve is also provided with a pressure balancing arrangement for counteracting the effects of pressure variation upstream or downstream of the metering surfaces on the relative positioning of the surfaces. The pressure balancing arrangement comprises one or more diaphragms operatively connected to the poppet and responsive to the pressure variations to generate forces which act in opposition to the forces exerted on the stem by the poppet in response to these same pressure variations.
The valve of the present invention is also provided with mechanical means for adjusting and/or calibrating the flow of gaseous media through the valve. The calibration mechanism is externally accessible so that the valve may be calibrated without disassembly or required access to the internal valve components. The valve may also be calibrated electrically by regulation and control of the current supplied to the electrically powered solenoid.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.